| name | performance |
| description | Optimize UI performance against Core Web Vitals — LCP, INP, CLS — with loading/code-split strategy, layout-shift prevention, and animation performance rules. Use when the user wants to improve speed, fix jank or layout shift, hit Web Vitals budgets, or make a UI feel fast on low-end devices. |
Skill: Performance
Make the UI fast and stable. Treat performance as an accessibility concern — slow/janky UIs fail low-end devices first.
Steps
- Read
workflows/performance.md (Core Web Vitals targets, loading strategy, layout-shift, animation perf, design-system runtime cost).
- Diagnose against budgets: LCP ≤ 2.5s, INP ≤ 200ms, CLS ≤ 0.1. Measure on a mid-tier mobile profile (throttle slow 4G + 4× CPU).
- Loading: render above-fold first (SSR/Server Components), code-split by route + heavy widget, lazy-load below-fold/behind-interaction (Astro islands / Qwik), preload one critical font weight, modern responsive images.
- Kill layout shift: size skeletons to final dimensions, reserve media space via
aspect-ratio (tokens/sizing.json), never inject content above existing.
- Animation: only
transform/opacity, will-change sparingly, 100–300ms (tokens/motion.json), honor prefers-reduced-motion. Prefer CSS state styling over JS for low INP; tree-shake/per-component imports.
Verification (definition of done)
- Lighthouse / field data meets LCP ≤ 2.5s, INP ≤ 200ms, CLS ≤ 0.1 on mid-tier mobile.
- First interaction stays responsive under slow-4G + 4× CPU throttle.
- Skeletons + media reserve space (zero CLS on load); no non-compositor animation in loops.
Kit location
This skill is part of ux-ui-agent-skills v2.4.0.
Resolve every relative path in the steps above against:
.agents/ux-ui-agent-skills/
(from repo root: /Users/tao.exe/Documents/skill/.agents/ux-ui-agent-skills)
Load .agents/ux-ui-agent-skills/CLAUDE.md when you need the full agent persona or Request Router.
